Find the keyhole. If your doorknob is accompanied by a lock, you should see a jagged slit on the outward-facing knob of the door. There should be a locking button on the inside part of the knob. The jagged slit is the keyhole. If you have a key for this door, it should fit perfectly into the keyhole and will allow anyone with that specific key to enter the premises.

Make sure that your key fits. Slide the key into the keyhole of the outer doorknob. If the key will not fit, flip it upside down and try again. The key may have one jagged edge and one smooth edge, or it may have multiple jagged edges. These jagged ridges along the length of the key are what match it to this particular lock. The more jagged the key, the tighter the security.

Lock the door from the outside. Close the door while you are standing outside. Slide the key into the keyhole and turn it clockwise as far as it will go. If you turn the key far enough, the door should lock. If this does not work, flip the key upside down and try again.

Lock the door from the inside. You don't need a key to lock most doors from the inside. Find the push-lock or twist-lock button on the inner doorknob.

Make sure that the door is locked. Try to turn the doorknob and push the door open. If the knob turns and the door opens, then you didn't lock the door. If the knob rattles, but does not turn, then you have locked the door.

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